And the believers should not all go out to fight. Of every troop of them, a party only should go forth, that they (who are left behind) may gain sound knowledge in religion, and that they may warn their folk when they return to them, so that they may beware

We see from this verse that Allah is showing that combative Jihād is not for everyone. If a group of people have been assigned to go for combative Jihād by their leader, the rest must not go. Rather their duty is to stay behind and study, in order to educate themselves and to educate others.

So from this verse, Allah split the people who participate in Jihād into two categories: One group goes to battle and the other stays behind to develop understanding of the religion in order to teach others. So even when combative Jihād has been called for, both those who go forth to combat and those who stay behind to develop understanding of religion are participants in Jihād. This verse makes those who stay behind and study the religion equal to those who go forth to battle, by saying: “their duty is to stay behind and study, in order to educate themselves and to educate others.”

In this verse Allah emphasized that not all the believers should go out to fight. This indicates that there is a decision to be made: who will go to fight and who will not? This implies existence of leadership who must first decide if it is necessary to go for fighting or not. It is not the case that people from here and there may issue a call to go for fighting, which will result in nothing but anarchy.Mu'adh ibn Jabal related:

Acquire knowledge because doing so is goodness, seeking it is worship, reviewing it is glorifying Allah and researching it is Jihād...

From this we can see that to learn the religion becomes more important than participation in battle, for it will elaborate for you all the beliefs and the rulings of all that Muslims must do in this life. To understand the rulings of the religion, including those related to Jihād, is essential and can only be accomplished by study and education. If someone has not studied comprehensively the rulings of Jihād, he will easily come to the conclusion that every issue that is raised entails combative Jihād, whereas this indeed is not the case.

Conscription

Participation in combative Jihād becomes assigned to an individual when he is ordered by the leader to be present in the line of fire:

Hence the Messenger of Allah said: “There is no migration (after the opening of Makkah), but Jihād and good intention. So when you are called to go forth in stopping aggression, then do so.”

This means when you are called out by your leader, and we have explained before who has the right to issue a declaration of war, the Imāmate leader, you must obey, as that is part of obedience to Allah, the Prophet and those in authority. And the condition for such a declaration of war is when the enemy suddenly attacks a land. In that case combative Jihād is appointed on its inhabitants and they must go forth to defend their nation from aggression.

Along with this it is incumbent on any group who seek to fight as soldiers in the way of Allah against aggression by unbelievers, to firstly pledge themselves to their leader—someone who fits the profile of Imāmate from knowledge, piety and effectiveness—who organizes the army. Thereafter they organize their ranks, and prepare them to fight. Setting forth when called is mandatory on the Muslim when: male; possesses sound reason; attained the age of maturity; is healthy and whose family possesses sufficient funds for what they need until he completes the duty assigned him by the leader.

Allah said, setting the rules for Jihād, “and the believers should not all go out to fight”

The verse begins with that statement to emphasize that not every person goes forth to battle, and it goes on to explain that “from every troop of them, a party only should go forth, that those who are left behind may gain some knowledge in religion and that they may warn their folk when they return tot hem that they may beware.”

Allah is showing that from every group, only a party of them goes forth. That means the army is to be taken from different citizens from various parts of the country, “from every group of them,” and today means volunteers or recruits who have been assigned and trained go forth to fight, while the rest of the citizens remain behind to train and educate themselves.

Not unto the weak nor unto the sick nor unto those who can find naught to spend is any fault (to be imputed though they stay at home) if they are true to Allah and His messenger.

This verse means there is no obligation on those who have a weak personality, or a radical mentality, nor on those who have no talent, to go forth, for war will not be good for them. This indicates only those persons selected by the Ruler or his appointed leaders should go forth; not those who might commit rash actions because of excessive emotional zeal nor those who are mentally ill and might commit crimes like the throwing of bombs, suicide attacks and so forth.

As Ibn Qayyim al-Jawzīyyah said in Zād al-ma‘ād:The Prophet said:

المجاهد من جاهد نفسه في طاعة الله والمهاجر من هجر الخطايا والذنوب

The fighter is the one who fights himself in obedience to Allah and the one who emigrates is the one who emigrates from iniquities.

The Jihād of the self is a prerequisite over the Jihād of the enemy in the open and initial basis for it.

Without a doubt, the one who didn’t fight his self (or someone who does not do what he is commanded and does not leave what has been forbidden goes to battle in the way of Allah) it is not possible for him to make combative Jihād against the external enemy. How is this it possible for him to fight his [external] enemy, when his own enemy which is right beside him, dominates over him and commands him? So as he did not wage war on the [internal] enemy of Allah, it is even more impossible for him to set out against the enemy until he fights himself in order for departure.

لَيْسَ عَلَى الْأَعْمَى حَرَجٌ وَلَا عَلَى الْأَعْرَجِ حَرَجٌ

There is no blame for the blind, nor is there blame for the lame, nor is there blame for the sick (that they go not forth to war).

Surprise Attack

When the enemy suddenly arrives in a place which the Muslims reside, it is obligated for the inhabitants to go out and fight them and it is not allowed for anyone to be exempt from this obligation.

Age Requirement

Ibn 'Umar said, “I was presented to the Messenger of Allah at the time of the battle of Uħud when I was fourteen years of age, and he didn’t give me permission to fight.”This is because Jihād is not obligated except on the one who has reached the appropriate age.

And covet not the thing in which Allah hath made some of you excel others. Unto men a fortune from that which they have earned, and unto women a fortune from that which they have earned. (Envy not one another) but ask Allah of His bounty. Lo! Allah is ever Knower of all things

It is reported by 'Ikrimah that some women inquired about Jihād and other women said, “We wish that Allah grant us a portion of the reward the military expeditions receive from the reward of what the men share.”This does not prevent women from going out to treat the wounded.

It is reported that the Prophet was out on a military expedition and Umm Salīm was with him and other women from the al-Anšār. They were giving water to the fighters and treating the wounded.

Parents’ Permission

In the case of a major, obligatory combative Jihād, the parents’ permission is not required, but as far as the voluntary combative Jihād, their permission is a must but from just one if the other has passed away.

Ibn Mas'ūd related:I asked the Messenger of Allah which action is most loved to Allah and he said, “Prayer in its time. Then I said, “then what,” and he said, “Being good to your parents.” Then I said, “what after that?” He said, “Jihād in the way of Allah.”

Ibn '`Umar said:A man came to the Prophet and asked permission for combative Jihād and he said, “Are not your parents alive?” He said, “Yes.” Then he said, “Then ask them first, then fight.”

One does not go out in Jihād except if he has completed providing for the needs of his family and the service of his parents. For this is the prerequisite of Jihād; even more it is the best Jihād.